1. Technical Field
The present invention relates generally to the field of semi-conductor manufacturing and, more specifically, to a method, when using a frequency doubling resist material to form sub-lithographic features, of trimming the ends of the printed features.
2. Background Art
Manufacturing of semiconductor devices is dependent upon the accurate replication of computer aided design (CAD) generated patterns onto the surface of a device substrate. The replication process is typically performed using lithographic processes followed by a variety of subtractive (etch) and additive (deposition) processes.
Photolithography, a type of lithographic process, is used in the manufacturing of semiconductor devices, integrated optics, and photomasks. The process basically comprises: applying a layer of a material that will react when exposed to light, known as a photoresist or, simply, a resist; selectively exposing portions of the photoresist to light or other ionizing radiation, i.e., ultraviolet, electron beams, X-rays, etc., thereby changing the solubility of portions of the material; and developing the resist by washing it with a basic developer solution, such as tetramethylammonium hydroxide (TMAH), thereby removing the non-irradiated (in a negative resist) or irradiated (in a positive resist) portions of the layer.
Conventional positive and negative tone photoresists are characterized by a dissolution curve in which there is a single transition from a first dissolution rate to a second dissolution rate as the resist is exposed to varying levels of actinic radiation. In a positive resist, the initially unexposed resist is insoluble in developer, while the exposed resist becomes more soluble as the exposure dose is increased above a threshold value. For a negative resist, similar behavior is observed, except that the initially unexposed resist is soluble in developer, and the exposed area is rendered insoluble. By means of this differential solubility between the exposed and unexposed resist areas, it is possible to form a pattern in the resist film. This pattern can be used to form integrated circuit devices, and is currently a critical component in their manufacture.
In an ideal situation, the exposure tool would only allow the radiation to hit the resist material in the areas of the mask that are clear, thus providing sharp edges for the lines and spaces. However, diffraction patterns are formed at the edges of the clear areas, resulting in partial exposure of the resist in those areas. Certain patents have taken advantage of this phenomenon, such as U.S. Pat. No. 4,568,631 issued to Badami et al. on Feb. 4, 1986 and assigned to the assignee of record for the present invention, which discloses utilizing a positive resist and an additive for image reversal in order to create thin resist lines only in the areas where light intensity has been reduced by diffraction effects. However, this procedure uses a resist with conventional positive and negative tone dissolution responses and requires two separate expose and develop operations to form a resist image from the edge of a reticle image.
It is desirable, therefore, to devise new mechanisms of resist response such that conventional optical lithography can be extended to smaller feature sizes without developing new tools and reticles. Additionally, as these new tools and reticles are eventually developed and implemented, these new resist approaches would remain applicable as a further extension of lithographic capability.
Previously, a new frequency doubling hybrid photoresist was developed to extend photolithography to a smaller feature size. The hybrid resist uses the edges of the mask shapes to define the feature spaces in the resist. However, because the edge of the mask shape continues around the entire perimeter of the shape, these edge defined spaces are all linked together. Thus, features created using hybrid resist are "linked" together. While this linking is acceptable in some situations, such as in the formation of shallow trench isolations, the linking can be unacceptable in other situations where the linking can cause unwanted shorting.